Influence of RAFT end‐groups on the water swelling of poly( N ‐propyl methacrylate)

Abstract

The immersion of poly(n-propyl methacrylate), PPMA, films (ca., 425 nm) in water induces swelling that is measured in-situ using spectroscopic ellipsometry. Unexpectedly, the end group of the PPMA resulting from the reversible addition-fragmentation chain transfer (RAFT) polymerization impacts the temperature dependence of swelling, despite their relatively high molecular weights (ca., 30 kDa). At 25 8C, dithiobenzoate terminated PPMA (PPMA-DB) leads to significantly less swelling (5.6 vol %) than the dodecylsulfanylthiocarbonyl terminated PPMA (PPMA-DD, 9.0 vol %). These PPMA films swell significantly more than expected due to a common carboxylic acid end group. As temperature is increased, the swelling for PPMA-DB increases and that for PPMA-DD decreases, with a crossover at approximately 35 8C-40 8C where the swelling is indistinguishable between the two polymers. The swelling kinetics exhibit two stages: an initial rapid swelling within the first minute of immersion and then a slow increase in thickness over multiple hours. The water contact angle of PPMA-DB increases on heating, while the water contact angle of PPMA-DD is invariant. This difference in the temperature dependence of the hydrophobicity is consistent with that for swelling. These results illustrate the potential unexpected consequences of residual RAFT fragment end groups on physical properties of polymers even at relatively high molecular weights.